Method of connecting pipe-in-pipe structures

ABSTRACT

A method of forming a structural member by connecting end-to-end first and second tubular structures, with each tubular structure having an inner tubular member (1a, 1b) located within a co-extending outer tubular member (3a, 3b) . The inner tubular member (1a, 1b) is longer than the outer tubular member (3a, 3b). The inner and outer tubular members (1b, 3b) of the second tubular structure are movable relative to each other in an axial direction. The method includes joining together adjacent ends of the inner tubular members (1a, 1b) of the first and second tubular structures respectively, displacing the outer tubular member (3b) of the second tubular structure axially relative to the inner tubular member (1b) of the second tubular structures until it abuts against and is contiguous with the outer tubular member (3a) of the first tubular structure, and at least partially overlies the inner tubular member (1a) of the first tubular structure, and then joining together the abutting outer tubular members (3a, 3b) to thereby form the structural member.

The invention relates to a method of connecting together tubular memberswhich have co-extending elongate members therewithin. The invention willbe described with reference to its use in connecting together the pipesof a pipeline and tubular structural members such as those typicallyutilised in the offshore oil and gas industry.

With the growth of the offshore oil and gas industry, it is necessary toinstall prefabricated pipelines between offshore oil and gas fields andonshore processing facilities and between different offshore locations.These may be for processing purposes or for loading the oil or gas intoshuttle tankers from offshore loading buoys.

To meet the need to install pipelines with a high degree of reliability,it is possible to use barges and vessels with several aligned welding,inspection and coating stations that can fabricate a pipeline from alarge number of short lengths of coated pipe, typically 12 metres long.The welded joints of the pipeline have then to be inspected and coatedbefore the pipeline is lowered to the sea bed down a long curved stingeror ramp.

A further alternative is to fabricate, at a suitable shore site, abundled assembly formed of a pipeline or group of pipelines housedwithin a large diameter outer steel pipeline, known as a carrier pipe.The void space within the carrier pipe and around the pipelines housedwithin it is sealed to prevent the ingress of water.

A particular trend is to install pipelines which are to operate at hightemperatures and/or pressures, which give rise to significant integralforces when the pipeline is prevented from expanding or allowed onlypartial expansion.

In use, submarine pipelines are often required to maintain certain fluidtemperatures, preventing adverse affect such as hydrate formation or waxdeposition. Thermal insulation of the pipelines helps to avoid this andis currently achieved by the application to their external surface asuitable insulation material. The materials are presently applied byvarious processes including extrusion, impingement, wrapping andcasting.

For high fluid temperatures and pressures, provision of thermalinsulation is achieved by housing the fluid carrying pipeline orflowline, as it is commonly termed, within an outer and additionalpipeline. This is known as a "pipe-in-pipe" system. The annulus betweenthe inner flowline and outer pipeline may be filled with thermalinsulant having low order thermal conductivity, such as polyurethanefoams, mineral wool or ceramic microspheres. Alternatively, the annulusgap may be a vacuum or full of gas.

Also a particular trend is to install pipelines which are to operate athigh pressures and high temperatures, where if the pipeline is preventedfrom expanding or is allowed only limited expansion, would lead to thepipeline experiencing induced strains beyond the elastic limit of thematerial from which it is made.

To form elongate tubular structural members such as the tethers onTension Leg Platforms, short lengths of "tube-in-tube" structuralmembers are adjoined end to end by either butt welding, internal orexternal collars secured by fillet welds or mechanical means such asscrewed ends or splined connectors.

Where pipe-in-pipe pipelines or tube-in-tube structural members areconstructed, it is necessary to complete the connection together of twoinner pipes or two inner structural members prior to connecting togetherthe two outer pipes or two outer structural members. Where the annulusbetween the inner pipe or tube and the outer pipe or tube containsmaterials or mechanical apparatus which prevent relative longitudinalmovement of the two co-extending pipes or structural members, there is atechnical problem faced in closing the gap between the ends of the outerpipes or structural member which is required for access to connect theinner pipes or structural members.

A solution has been proposed in our co-pending patent application GB9623131.1. This method of joining pipelines utilises an intermediatemember which is screwed on a thread relative to a first pipe section inorder to bring the intermediate member into abutment with the secondpipe section. Once the intermediate and second pipe members are broughtinto abutment with each other, they can be welded together. This methodenables the assembler to access the interior flowline before fixingtogether the external tubular members, whilst eliminating the need forfillet welds which are prone to failure and which are hard to x-ray.

A solution to the problem of joining pipe-in-pipe or tube-in-tubemembers is now proposed for situations where the inner and outer pipe ortube members are able to move relative to each other in the longitudinaldirection.

According to the invention there is therefore provided a method offorming a structural member by connecting end-to-end first and secondtubular structures, each tubular structure comprising an inner tubularmember located within a co-extending outer tubular member, the innertubular member being longer than that of the outer tubular member sothat at least a portion of one end of the inner tubular members projectsfrom the corresponding end of the outer tubular member, in which theinner and outer tubular members of the second tubular structure aremovable relative to each other in an axial direction, the method beingcharacterised by the steps of joining together adjacent ends of theinner tubular members of first and second tubular structuresrespectively, displacing the outer tubular member of the second tubularstructure axially relative to the inner tubular member of the secondtubular structures until it abuts against and is contiguous with theouter tubular member of the first tubular structure, and at leastpartially overlies the inner tubular member of the first tubularstructure, and joining together the abutting outer tubular members tothereby form the structural member.

Preferably the inner members are joined together by welding.

The welded connection between the inner members is preferably checked,by, for example, x-ray, before the second outer member is displaced.

In a further embodiment the first tubular structure comprises astructural member formed from a plurality of individual tubularstructures previously joined together.

Preferably the method further comprises the step of sliding a sleeveonto the inner tubular member of the first tubular structure, whereinthe sleeve is of a shorter length than the portion of the inner tubularmember projecting from the outer tubular member, displacing the sleeveaxially relative to the inner tubular member until it abuts against andis contiguous with the outer tubular member of the first tubularstructure and overlies a part of the inner tubular member of the firsttubular structure.

In a preferred embodiment of the invention the steps of removablyattaching an end of a tubular dummy member to an end of the innertubular member of the first tubular structure, said dummy member beinglocated within a co-extending outer tubular member, displacing the outertubular member axially along the dummy member until it is whollypositioned on the adjacent inner tubular member and removing the dummymember.

Preferably the tubular structures further comprise an intermediatetubular member surrounding and co-axial with the inner tubular memberand located within and co-axial with the outer tubular member, saidintermediate member being shorter than the inner tubular members, themethod of connecting said structures further comprising the step ofdisplacing the intermediate tubular member of the second tubularstructure together with the corresponding outer tubular member axiallyalong the inner tubular member of the second tubular structure until theintermediate tubular members abut and are contiguous with each other,and joining the abutting intermediate tubular members and then abuttingand joining the outer tubular members.

The invention also provides an inner pipe comprising a plurality ofcontiguous inner tubular members and an outer pipe comprising aplurality of contiguous outer tubular members, said outer and innerpipes defining an annulus therebetween.

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawings inwhich:

FIG. 1 shows a side sectional elevation of two adjacent inner pipes andparts of the surrounding outer pipe;

FIG. 2 shows the sectional elevation of FIG. 1 with the outer pipesadjoining and welded together;

FIGS. 3 to 13 illustrate the method steps according to the presentinvention; and

FIG. 14 and 15 show similar side sectional elevations to those of FIGS.1 and 2 but with an intermediate pipe located between the inner pipesand the outer pipes.

Referring to FIG. 1, there are shown two tubular members to be joined toform a tubular structure, in this illustration being pipes for aflowline. These pipes comprise inner pipes 1a, 1b, joined end-to-end bya welded connection 2, to form a flowline, and an outer pipe 3a, 3b,within which the inner pipes 1a, 1b are located. Each of the outer pipes3a, 3b terminates short of each end of the respective inner pipes 1a,1b. Supporting each end of the outer pipes 3 on their respective innerpipes 1 are a pair of bulkheads 4, 5, one positioned at each end of theouter pipe. The bulkheads 4, 5 are substantially annular disks ofrubber/steel or other suitable material which is capable of being slidalong the inner pipe 1. The bulkhead 5 defines a leading end of theouter pipe 3 and is attached to the outer pipe 3 by means of a metallicbacking plate 6, such that the bulkhead 5 does not actually lie withinthe outer pipe 3 so as to avoid damage to the bulkhead when the outerpipes are joined by welding. Bulkhead 4, on the other hand, ispositioned within the outer pipe 3. The bulkheads 4, 5 fix the positionof the outer pipes 3 relative to the inner pipes 1. In a preferredembodiment of the invention the annular spaces defined between theco-axial outer pipes 3 and inner pipes 1 are filled with a thermalinsulant 7. Alternatively the annular spaces may be evacuated or filledwith an insulating gas.

The perimeter of the leading edge of the bulkhead 5 is preferablybevelled with a 30° bevel. The ends of each outer pipe 3a are alsosimilarly bevelled.

Two tubular structures are placed with their inner pipes 1a, 1b adjacenteach other and are joined, typically by welding together as shown inFIG. 3. Where inner pipes 1a, 1b are adjoined by welding, the firstwelded connection 2a between inner pipes 1a and 1b is x-rayed to checkthe integrity of the weld. If the connection 2a is satisfactory, outerpipe 3b is slid axially along inner pipe 1b and partially onto innerpipe 1a until the leading bulkhead 5 of outer pipe 3b abuts the bulkhead4 of outer pipe 3a. The bevel on the leading edge of the bulkhead 5eases the feeding of the bulkhead 5 into the adjacent outer pipe 3a.When in position, the bevelled ends of the outer pipes 3a, 3b are weldedtogether to form a welded connection 8 as shown in FIG. 2. The presenceof the backing plate 6 facilitates welding and prevents damage to thebulkhead 5 and any insulating materials 7 from the heat created by thewelding process.

In commercial applications it may be preferable to join together fourpipes at a time, e.g. 1a, 1b, 1c and 1d as shown in FIG. 3. Thus as theouter pipes 3a and 3b are joined, at the same time a further inner pipe1e is located adjacent pipe 1d and welded thereto, as shown in Fig.4.

When the welded connection 2b has been x-rayed and found to besatisfactory the outer pipe 3c is shuttled along the welded pipelineuntil it abuts outer pipe 3b and is welded thereto to create weldedconnection 8b. A new inner pipe if carrying outer pipe 3f is welded tothe end of inner pipe 1e as shown in FIG. 5. This process of welding,x-raying and shuttling continues as shown in FIGS. 6 to 9.

As the outer pipes 3 are shorter than inner pipes 1, eventually thisdiscrepancy needs to be accommodated, as follows.

The distance X between the bulkheads 4 and 5 on adjacent pipes istypically 400 mm. Obviously, as each outer pipe 3 is welded to itsneighbour the gap Y between the welded length of outer pipes 3 and thenext outer pipe will grow once this gap Y reaches a predeterminedlength, up to the full length of outer pipe 3a, but typically 8 metres,as in FIG. 11, a dummy pipe 10, supporting an outer pipe 3x is pushfitted onto the end of the last inner pipe 1h in the welded flowline.The outer pipe 3x is shuttled along until it is wholly located over theadjacent inner pipe 1h. The dummy pipe 10 is removed as shown in FIG. 12and the whole procedure starts again with the addition of a new pipemember 1I,3I to the end of the flowline, the x-raying of weldedconnection 2f prior to the shuttling of outer pipe 3h, followed by thewelding of outer pipe 3h to 3g.

Sometimes outer sleeves may be used which are shorter than usual, egouter pipe 3f which is shorter than outer pipes 3a, 3b, 3c, 3d, suchthat when it is slid to abut and connected to outer pipe 3e, the lengthof the protrusion of inner pipe 1e beyond outer pipe 3f is the samelength as that of inner pipe 1a to outer pipe 3a. Such sleeves may beused without the benefit of a dummy pipe and are slid straight onto theexposed length of inner pipe. This means that the gap Y will grow fasterthan if all the outer pipes are of the same length and the use of adummy pipe 10 will be more frequently needed and enables the operator tocontrol where the extra outer pipes 3x are located.

Thus the ability to x-ray each pipe joint 2 before the outer pipe ismoved over it and welded to the adjacent outer pipe means that theintegrity of each welded connection 2 is ensured obviously anyappropriate means of testing the welded connection, other than by x-ray,can be used.

Although for convenience only a single flowline made up of inner pipes1a, 1b, 1c. . . 1I. is illustrated, a plurality of parallel pipes orother elongate members maybe housed within the outer pipes. Furthermore,whilst the tubular members described in the aforegoing description arecylindrical pipes for a flowline, the tubular members could have anychosen cross section.

Yet another embodiment of the invention is shown in FIGS. 14 and 15.Referring to FIG. 14 there are shown two tubular structures, such aspipes for a flowline. These structures comprise inner pipes 1a, 1b,joined end to end, preferably by a welded connection 2, to form aflowline, outer pipes 3a, 3b and intermediate pipes 11a, 11b. The innerpipes 1a and 1b are located within the intermediate pipes 11a, 11b. Eachof the outer pipes 3a, 3b and intermediate pipes 11a, 11b terminatesshort of each end of respective inner pipes 1a, 1b. The intermediatepipes 11a, 11b are adjoined by mechanical connection, welding or othersuitable means. When in position, the bevelled ends of the outer pipes3a, 3b are again welded together to form a welded connection 8 as shownin FIG. 15. outer pipes 3a, 3b, intermediate pipes 11a, 11b, bulkheads 4and 5 and inner pipes 1a, 1b are assembled utilising the shuttlingtechnique as described above.

Again a plurality of parallel pipes or other elongate members may behoused with the intermediate pipes.

The annular gaps between the inner and intermediate pipes and the outerand intermediate pipes may be filled with thermal insulant, air, a gassuch as nitrogen or carbon dioxide or left empty or evacuated.Additional fluid may be passed through the gap between the inner pipeand the intermediate pipe to provide a means of either heating orcooling the inner pipes.

Where access is required to join the intermediate pipes by welding orother mechanical means, the intermediate pipes may protrude beyond theends of the outer pipes. The gap between the ends of the outer pipes maythen be closed by a short length of outer pipe made up in two halves, ashort length collar of length greater than the gap between the outerpipes, the collar being initially positioned inside or on the outside ofthe outer pipe. To facilitate assembly of the outer pipes in this way,the bulkheads 4 and 5 may be positioned at the end or at a distance backfrom the ends of the outer pipes to facilitate welding. The gap betweenbulkheads 4 and 5 may be either filled with thermal insulant or air,left empty, or purged with gas such as nitrogen or carbon dioxide.

As an alternative to utilising the backing plate 6, the bulkhead 5 maybe positioned within the outer pipe 3 to prevent damage during weldingof the outer pipes. The gap between the bulkhead 4 and 5 then may beeither filled with thermal insulant or air, left empty, or purged withgas such as nitrogen or carbon dioxide.

A further alternative is to pass cooling or heating fluid, typicallywater, through the gap over the full or part length of the pipeline. Forhigh temperatures and/or pressures the gap between the inner pipe andthe intermediate pipe may be sufficient to allow expansion or partialexpansion of the inner pipe, laterally, in a spiral formulation orlongitudinally beyond the ends of the pipeline, or a combination of allthree.

Although the foregoing describes the joining of the outer pipes welding,they may alternatively be joined by mechanical connectors, or othersuitable means.

What is claimed is:
 1. A method of forming a structural member byconnecting end-to-end a plurality of tubular structures, each tubularstructure comprising an inner tubular member (1a, 1b, 1c) located withina co-extending outer tubular member (3a, 3b, 3c), the inner tubularmember (1a, 1b, 1c) being longer than the outer tubular member (3a, 3b,3c) so that at least a portion of one end of the inner tubular members(1a, 1b, 1c) projects from the corresponding end of the outer tubularmember (3a, 3b, 3c), in which the inner and outer tubular members of thetubular structures are movable relative to each other in an axialdirection, the method being characterized by the steps of joiningtogether simultaneously abutting adjacent ends of outer tubular members(3a, 3b,) of one pair of adjacent tubular structures and abuttingadjacent ends of the inner tubular members (1b, 1c) of another pair ofadjacent tubular structures, displacing the outer tubular member (3c) ofa tubular structure adjoining the one pair axially relative to its innertubular member (1c) until it abuts against and is contiguous with theouter tubular member (3b) of the one pair of tubular structures, and atleast partially overlies the inner tubular member (1b) of the one pairof tubular structures.
 2. A method as claimed in claim 1 in which theinner tubular members (1a, 1b) are joined together by welding.
 3. Amethod as claimed in claim 2 in which the welded connection (2) betweenthe adjacent inner tubular members (1a, 1b) is checked, by, for example,x-ray, before the outer tubular member (3b) is displaced.
 4. A method asclaimed in claim 1 in which the first tubular structure comprises astructural member formed from a plurality of individual tubularstructures (1a, 2a; 1b, 3b) previously joined together.
 5. A method asclaimed in claim 1 further comprising the step of sliding a sleeve (3f)onto the inner tubular member (1a, 1b, 1c, 1d, 1e) of the first tubularstructure, wherein the sleeve (3f) is of a shorter length than theportion of the inner tubular member (1a, 1b, 1c, 1d, 1e) projecting fromthe outer tubular member (3e), displacing the sleeve (3f) axiallyrelative to the inner tubular member (1a, 1b, 1c, 1d, 1e) until it abutsagainst and is contiguous with the outer tubular member (3e) of thefirst tubular structure and overlies a part of the inner tubular member(1a, 1b, 1c, 1d, 1e) of the first tubular structure.
 6. A method asclaimed in claim 1 further comprising the steps of removably attachingan end of a tubular dummy member (10) to an end of the inner tubularmember (1a, 1h) of the first tubular structure, said dummy member (10)being located within a co-extending outer tubular member (3x),displacing the outer tubular member (3x) axially along the dummy member(10) until it is wholly positioned on the adjacent inner tubular member(1h) and removing the dummy member (10).
 7. A method as claimed in claim1 in which the tubular structures further comprise an intermediatetubular member (11) surrounding and co-axial with the inner tubularmember (1) and located within and co-axial with the outer tubular member(3), said intermediate member (11) being shorter than the inner tubularmembers (1), the method of connecting said structures further comprisingthe step of displacing the intermediate tubular member (11b) of thesecond tubular structure together with the corresponding outer tubularmember (2b) axially along the inner tubular member (1b) of the secondtubular structure until the intermediate tubular members (11a, 11b) abutand are contiguous with each other, and joining the abuttingintermediate tubular members (11a, 11b) and then abutting and joiningthe outer tubular members (3a, 3b).
 8. A method as claimed in claim 1 inwhich the tubular structures comprise a plurality of parallel innertubular members, each of which is joined to adjacent ends of innertubular members in adjoining tubular structures.
 9. A pipe-in-pipestructural member comprising a plurality of tubular structures connectedend-to-end by the method claimed in claim 1 to provide an inner pipecomprising a plurality of contiguous inner tubular members (1a, 1b, 1c,1d, 1e) and an outer pipe comprising a plurality of contiguous outertubular members (3a, 3b, 3c, 3d, 3e), said outer and inner pipesdefining an annulus therebetween.